1. Field of the Invention
This invention relates to a recording medium for use in magnetooptic memory devices, magnetic recording systems, display elements, etc.
2. Description of the Prior Art
With the recent expansion of information-oriented society, there is demand for a memory device or recording system which has a simple structure and a large capacity of information recording. At the same time, extensive research and development are made for recording media capable of high density and large volume recording.
An example of such recording media is the magnetooptic recording medium. It comprises a thin recording layer such as a polycrystalline film of, e.g. MnBi or MnCuBi, an amorphous film of, e.g. GdCo, GdFe, TbFe, DyFe, GdTbFe or TbDyFe, a single-crystal film of, e.g. GIG (gadolinium-iron-garnet). Information recorded as the inversion of magnetization with light and heat energy or the like in the recording medium is read by utilizing a magnetooptical effect such as magnetic Kerr effect or Faraday effect.
Among the above thin films constructing recording layers, the amorphous film is lately considered to be superior as a magnetooptical recording medium in view of the film-forming property of the material for making a thin, large film at around room temperature, the light and heat energy efficiency for writing signals therein, and the S/N ratio in reading the written signals. In particular GdTbFe is best suited for magnetooptical recording media because of its large Kerr rotational angle and approximate Curie temperature of 150.degree. C.
However, amorphous magnetic materials including GdTbFe as a typical example have generally a drawback of low corrosion resistance; that is, these materials, on exposure to the atmosphere or water vapor, undergo deterioration of magnetic characteristics, and eventually, are oxidized completely and turns transparent.
Corrosion is a very critical problem not only for the recording media made of the above amorphous magnetic materials but also for those made of other magnetic or non-magnetic materials. The corrosion of recording materials will make the writing of information impossible and at the worst it would erase even the important recorded information.
There have so far been known recording media comprising a recording layer coated with a protective layer of, for example, SiO.sub.2 or SiO and those comprising a pair of recording layers which are supported each on a substrate and bonded to be in contact with each other. However, both types of recording media are not so effective for preventing the corrosion of the recording layer, though effective for protecting from such damages as scratches. For the purpose of preventing the recording layer from oxidation, another type of recording medium has been proposed. This comprises a pair of recording layers which are supported each on a substrate, bonded together through a spacer with the recording layers facing each other, and contains an inert gas sealed in the inner space. However, this type of recording medium is disadvantageous in that the production thereof requires many steps resulting in high cost and the hollow structure thereof makes the medium fragile to a shock when the substrates are made of glass.